1. Field of the Invention
The present invention relates to a mirror mechanism for use in a single-lens reflex camera, and more particularly to a mirror descent spring for use in said mirror mechanism.
2. Description of the Prior Art
In single-lens reflex cameras there is already known a mirror mechanism allowing a manual ascent and descent of miror in order, for example, to allow replacement of batteries for exposure meter accommodated in the mirror chamber. In such mechanism the mirror, being provided with a descent spring for allowing manual displacement thereof, ascends upon shutter release by the force of drive mechanism and against the force of said mirror descent spring, and descends by the force of said spring upon descending function of the drive mechanism.
Apart from the quick-return function performed in the above-mentioned manner, the mirror is structured to be manually liftable for example for replacement of batteries mentioned above.
In case of a conventional mirror descent spring the axis of rotation of mirror or a separate axis provided on a mirror-supporting frame is inserted into a coiled portion of a torsion spring of which one end is fixed on the camera body while the other end engages with a part of said mirror.
In such structure wherein the axis of rotation of mirror is inserted into said coiled portion of spring, the photographing light beam may be partly blocked if the coiled portion of spring is enlarged since said axis of rotation is located close to the path of said light beam. The resulting limitation on the volume of said coiled portion renderes it impossible to reduce the spring constant beyond a certain limit, giving rise to a larger energy required for the drive mechanism to elevate the mirror. Also such mechanism is associated with the following drawback.
Referring to FIG. 1, A represents the drive torque generated by the mirror drive mechanism to elevate the mirror;
B represents the mirror descent torque generated by a conventional mirror descent spring;
C represents the load of a hooking lever which generally functions to hold the first blind or forward curtain of shutter by means of a hook and which transmits the completion of mirror ascent to the shutter mechanism (not shown) to start the movement of said first blind; and
D represents B+C wherein A&lt;D in the vicinity of the photograph-taking position so that the rotating system including the mirror drive mechanism and the mirror is decelerated, being subjected to a braking torque.
The value of C represents the force for unhooking the first blind of shutter, and it is necessary to select a large safety factor to achieve secure unhooking of the first blind even under eventual fluctuation of the value C since it involves various uncertain factors such as the change in the force of spring for driving the first blind or in the frictional force of hooking or the hooking efficiency. For this reason the value A should be increased in the vicinity of the mirror ascent state. Such change in the value A without altering the same in the vicinity of the mirror descent state signifies a smaller inclination for the curve A. However, since the curve A is already designed with a slope as small as possible, such change is only possible by increasing the value A throughout the entire curve, the mirror ascent and descent states inclusive. Such will inevitably result in a higher load in the film advancing operation as the drive torque for mirror ascent is developed by charging of the mirror drive mechanism at said film advancing operation.